FIELD OF THE INVENTION
The invention lies in the field of applied thermodynamics. Specifically, the present invention relates to a method for cooling a structural part of a gas turbine plant and to a corresponding gas turbine plant. The gas turbine plant has a turbomachine shaft, a compressor and a cooling device. The cooling device is formed with a first duct for a main air stream and a second duct for a partial air stream. The cooling device thereby utilizes the main and the partial air streams which are compressed by the compressor of the gas turbine plant. One end of the first duct of the cooling device opens directly or indirectly into a compression region of the compressor. The term "compression region" as used herein means, in particular, respective stages of a turbocompressor. According to the instantly disclosed invention, however, the "compression region" also includes the region downstream of the compressor in the direction of flow, such as the outlet diffuser.
In order to effectively increase the efficiency and the power of gas turbines, it is an object of continuous development to raise the turbine inlet temperature. The highly heated gas which enters the first turbine stage and flows out of the combustion chamber subjects this stage to particularly severe attack. Highly heat-resistant metallic materials allow inlet temperatures of about 600.degree. C. in stationary gas turbine plants, and in the case of aircraft engines they may even be about 900.degree. C. If higher working temperatures are required, at least the first turbine stage must be cooled. This is all the more necessary because the turbine stage itself also undergoes corrosion stresses due to the aggressiveness and oxygen content of the hot combustion gas and due to the centrifugal stress on the turbine rotor. To cool the first guide blades and moving blades of the gas turbine, it is therefore known to extract a partial mass air stream from the compressed main mass air stream downstream of the compressor, and, by passing the combustion chamber, feed the the partial mass air stream directly to cooling ducts of the guide blades via the housing and to the moving blades of the first rows through the rotor. The blades can then be cooled by convection cooling, film cooling or else transpiration cooling to such temperatures as ensure that the blades have an acceptable useful lifetime.
Of course, the partial mass air stream branched off from the main mass air stream causes the efficiency of the gas turbine plant to be impaired. The size of the branched-off partial mass air stream must therefore be dimensioned in such a way, that on the one hand, sufficient cooling, but, on the other hand, also high efficiency is achieved. For this purpose, it is also necessary for pressure losses to be minimized. For example where the partial air stream is concerned, a pressure loss occurs because delivery conduit losses and pressure losses due to cooling ducting occur. It is therefore necessary, after cooling, to feed the partial mass air stream back to the hot working gases, then flowing out of the combustion chamber, in such a way that the pressure of the partial mass air stream can still be utilized by the turbine.
It is also known that, in the case of large stationary plants, there is an additional external compressor which compresses the partial mass stream intended for cooling, in order to compensate pressure losses. However, the additional compressor likewise requires drive energy in the form of electrical current. The calculation of an efficiency of the gas turbine plant must then take this energy into account accordingly. The greater it is, the lower the efficiency of the gas turbine plant becomes.
German published patent application DE 33 10 529 A1 discloses the cooling of a gas turbine by means of compressor air which is further compressed by a centrifugal compressor. The centrifugal compressor is formed essentially by a rotating flow duct. The concept of compression by means of a centrifugal compressor necessitates considerable radial distances over which the air to be compressed must be ducted. Such radial distances are achieved with rotor discs which are typical of jet engines. U.S. Pat. No. 3,936,215 also describes a centrifugal compressor of an engine.